1 . Field of the Invention
The present invention relates to a manufacture method and apparatus for color filters by forming colored areas on a transparent substrate by applying ink by an ink jetting method, and to a liquid crystal display using color filters formed by such a manufacture method.
2. Related Background Art
Liquid crystal displays are mounted on various apparatuses such as personal computers, word processors, pachinko players, vehicle navigation systems and small TVs, and the demand for liquid crystal displays is increasing year after year.
Color filters of a liquid crystal display are formed by disposing pixels (colored areas) such as red (R), green (G) and blue (B) pixels on a transparent substrate in a predetermined pattern. A black matrix for light shading is formed around each pixel in order to raise a display contrast.
Conventional color filter manufacture methods include a dying method, a pigment dispersion method, an electrodeposition method and the like. In order to meet the requirements of cost-down, a printing method and an ink jetting method have been proposed. However, with the printing method, a transfer process for transferring a print impression and a drying process are repeated three times for R, G and B pixels to form color filters, so that a manufacture yield is low.
With the ink jetting method proposed, for example, in Japanese patent Laid-open Application No. 59-75205, coloring liquids containing R, G and B pigments are coated on a substrate and the coloring liquids are thereafter dried to form colored areas. With this ink jetting method, R, G and B pixels are formed by one process so that the manufacture process can be simplified considerably and the cost can be reduced greatly.
With a color filter manufacture method using the ink jetting method, the time taken by the coloring process can be shortened greatly by using ink jet heads having nozzles for respective colors. However, if a coloring area of a large color filter is divided into a plurality of scan areas and these areas are sequentially colored, the boundary area between adjacent scan areas has often a density different from other areas.
A relation between a coloring area and an ink jet head when the coloring area is divided into a plurality of scan areas and ink is applied thereto, is schematically shown in FIG. 1. In FIG. 1, 1a to 1c represent R, G and B color ink jet heads, 2 represents the whole coloring area, 3a to 3c represent scan areas, 4 represents a scan direction of the ink jet head, and 5 represents a shift direction of the ink jet head. In the example shown in FIG. 1, while a set of ink jet heads 1a to 1c of R, G and B is scanned in the scan direction 4 on each of the scan areas 3a to 3c of a substrate, ink from nozzles is applied to the coloring areas to form colored areas. If the substrate is so large that the whole coloring area 2 cannot be applied with ink by one scan operation in the scan direction, the ink jet heads 1a to 1c are shifted in the shift direction 5 and the scan operation is repeated. In this case, therefore, the whole coloring area 2 is divided into the scan areas 3a to 3c in correspondence with the shift positions of the ink jet heads 1a to 1c. Three sets of ink jet heads 1a to 1c corresponding to the three scan areas 3a to 3c may be prepared to color the whole coloring area 2 in a short time without any shift of the ink jet heads.
In either case, there are boundaries A between adjacent scan areas, and uneven color is observed on the boundary A and in its nearby area. The reason for this may be ascribed to that a pixel near the boundary A is different from a pixel remote from the boundary in the following points.
(1) There is a large time lag between when pixels in a scan area on one side of the boundary A are colored and when pixels in a different scan area on the other side of the boundary A are colored.
(2) A nozzle of the ink jet head which jets out ink toward a pixel in the boundary area in one scan area is positioned spaced apart far from a nozzle of the head which jets out ink toward a pixel in the boundary area in the adjacent scan area. Therefore, the physical amounts such as an ink drop let amount and an ink bombard position become different more than those of adjacent nozzles.
(3) If different ink jet heads are used for different scan areas, the physical amounts such as an ink droplet amount and an ink bombard position become different more than if the same ink jet head is used.
A color filter manufacture method using an ink jetting method is mainly classified into two methods. With one method, ink is applied to an ink absorptive resin layer to color it and form a colored area. With the other method, ink is applied to an area surrounded by a wall, and the ink is hardened to form a colored area. The mechanism why a difference of ink jet heads and nozzles described above makes uneven color, when these two methods are applied, may be ascribed to the following.
(First Method)
(a) It takes a longer wait time to perform a next process (ink drying process, resin layer hardening process) for the scan area first applied with ink than for the scan area thereafter applied with ink, when the ink jet heads are used by shifting. Therefore, the distribution of coloring agent changes and uneven color becomes visible.
(b) As the time lapses, ink components in a pixel impregnate into an adjacent pixel in the boundary area. Therefore, the distribution of coloring agent in adjacent pixels changes and uneven color becomes visible.
(c) The distribution of coloring agent in a pixel is affected by a difference in an ink bombard position and uneven color becomes visible.
(Second Method)
(d) A difference between the amounts of applied ink makes different thicknesses of colored areas which result in a density difference and uneven color becomes visible.
(e) A difference between ink bombard positions makes uneven thicknesses in a pixel which result in a density difference and uneven color becomes visible.